A carbonylation process of a methanol method is known as an industrial method for producing acetic acid. In this process, for example, methanol and carbon monoxide are reacted in the presence of a catalyst in a reaction vessel to produce acetic acid. The reaction mixture is evaporated in an evaporator, and the vapor phase is purified in a lower boiling point component removal column and subsequently in a dehydration column so that product acetic acid is prepared. Alternatively, product acetic acid is prepared via a higher boiling point component removal column subsequent to the dehydration column, and further, a product column.
In such an acetic acid production process, formic acid is produced as a by-product in the reaction vessel. The minimum amount of formic acid is favorable because the formic acid reduces the purity of product acetic acid. Patent Literatures 1 and 2 disclose that: formic acid is formed through the reaction of carbon monoxide with water; and therefore, the formic acid concentration in product acetic acid can be lowered by controlling a water concentration in a reaction medium to a low level. However, there is the problem that a catalyst becomes unstable if the water concentration in the reaction medium is decreased.